Supported metal catalysts are typically made by impregnating a suitable support material with a catalytically active metal or with its precursor. For example, catalysts for use in the production vinyl acetate monomer (VAM) by the reaction of ethylene, acetic acid and oxygen are made by impregnating a support such as silica or alumina with a compound of a Group VIII noble metal such as palladium together with a gold compound and an alkali metal salt, typically in the form of an acetate, the palladium and gold compounds being converted to catalytically active state.
In early examples of fixed-bed catalysts for use in the production of VAM, palladium and gold were distributed more or less uniformly throughout the support, for example, U.S. Pat. No. 3,743,607. Since gaseous reactants do not diffuse significantly into the large fixed-bed catalyst particles, much of the expensive catalytic metal components in the interior of the catalyst were not useful. Subsequently, shell-impregnated, fixed-bed catalysts were developed in which most of the catalytic metals were deposited onto an outer shell of the support particle. For example, Great Britain Patent No. 1,500,167 describes a catalyst in which at least ninety percent of the palladium and gold is distributed in that part of the support particle which is not more than thirty percent of the particle radius from the surface. The palladium and gold being at/near the surface are susceptible to loss through attrition.
In the preparation of shell-impregnated, fixed bed catalysts such as that described in GB 1,500,167 and EP-A-0 569 624, after impregnation of a support with a Group VIII noble metal solution, the noble metal is subsequently precipitated to the support by, for example, treatment with an aqueous solution of an alkali metal salt Such precipitated noble metal has limited mobility.
U.S. Pat. No. 4,677,084 describes a process for preparing attrition resistant catalyst, catalyst precursor and catalyst support particles and in particular silica-containing vanadium/phosphorus oxide catalysts. The catalyst, catalyst precursor or catalyst support particles are slurried in a solution of an oxide such as silica. The slurry is then spray-dried and calcined to produce microspheres. The process results in the formation of an oxide-rich layer at the periphery of each calcined microsphere.
There remains a need for an improved metal catalyst composition and in particular, a supported metal catalyst composition.